Over the past few weeks, complaints that something is causing iPhones to suddenly shut off with 30% battery have been mounting.

In a statement posted to its Chinese website on Friday, Apple restated that the issue is with hardware, and that it affects only certain iPhone 6S devices. iPhones with qualifying serial numbers can be brought into an Apple Store for a free battery swap.

But technicians and Apple Store employees have told Business Insider that the issue is very common, and that it seems to affect more models than just the 6S.

Apple has declined to comment several times when asked by Business Insider about whether the issue may affect other devices.

Complaints at Apple Stores are rising

Tomohiro Ohsumi/Getty

Apple Stores are seeing large numbers of complaints from customers concerned about why their devices are malfunctioning, several store employees have told Business Insider.

An Apple Store Genius employee in Manhattan said that 80% of its cases are currently a result of the bug. It appears to be affecting more iPhone models than the 6S, the employee said.

"We have been seeing a lot of customers for this particular issue, and I have personally dealt with the issue on not just the 6S, but the 6S Plus, which is not part of the program," said an Apple Store manager in another part of the US. "I've also seen the issue a lot for batteries outside of the manufacturing date."

Another US-based Genius said his store had placed 80 part orders for batteries after the replacement program was introduced, and that there weren't enough batteries on hand when Apple announced the program. "It really hasn't been that bad, to be honest," he told Business Insider.

LifehackerA different Apple Store employee in the US told Business Insider that they don't believe the reports of battery failures are related to iOS, but rather the failure is caused by a hardware problem, as Apple has stated. But even though the batteries shut down when they get to about 30% charge, Apple's battery diagnostics tool still says they are healthy, this employee said.

The employee also said they were getting several returns because of the battery issue but had no guidance from Apple on how to fix the problem until recently, so they ended up replacing phones instead of just the batteries, in most cases.

Another longtime Apple Store employee said customers frequently complain about battery issues after installing beta software or failing to install the newest version of iOS. "A lot of times it's older phones with consumed batteries or really old software," the person said.

One person who went to get their iPhone 6S fixed at an authorized Apple service provider in London said they were told the shop had a huge backlog of phones to fix and couldn't even provide a date for when it might be able to fix theirs.

Two bugs

Apple There is some debate about the cause of the problem. Apple says it's related to "a battery component that was exposed to controlled ambient air longer than it should have been."

But some have pointed to a recent iOS 10.1 update that was pushed to people's iPhones. People with the problem — including several Business Insider staff members whose phones are not covered by the replacement program — are seeing their batteries drain abnormally fast and switch off around the 30% charge mark.

A Chinese government watchdog group is also claiming the 6S hardware issue is affecting other devices, including the iPhone 6, 6 Plus, and 6S Plus.

Apple is replacing batteries in affected 6S devices free of charge, but it isn't talking about an apparent 10.1 bug. An Apple spokesperson declined Business Insider's repeated requests for comment on the 10.1 bug.

If you're having the issue, you can check if your device is covered under Apple's current replacement program, and if it is, you can have your battery swapped at an Apple Store.

If Apple says your phone is not eligible for a battery replacement but you are seeing the random shutdown bug, email kleswing@businessinsider.com.